I assume these are ex-VAT prices. If so that's about par for the course for 18mm, I reckon about £60 a sheet ex VAT delivered is pretty much the benchmark price.
I generally stay away from these pricing threads because they tend to gloss over or ignore completely the whole quality question. I'm making high end furniture and it would be rare for material costs to account for more than 20% of the finished cost, 10-15% would be closer to the average, so quality completely swamps price when it comes to sourcing materials.
This even applies to sheet goods. I'll generally use BB/BB birch ply as part of the substrate for veneering (it usually gets skinned both sides with MDF), so as well as void free ply of consistent thickness I'm also looking for exceptionally flat and stable ply. Birch ply comes from a huge swathe of Northern and Eastern Europe, with hundreds of manufacturers contesting the market. There's no doubt some of them would happily let quality standards slip in order to steal a small price advantage. But is that a product I want to have at the heart of my furniture? And the quality issue also extends to the retailers; ply and MDF needs to be stored very carefully to keep it flat. So I take the view that about £60 a sheet is sufficient to allow a manufacturer and retailer to provide excellent quality products, and having found suppliers who can achieve that (Timbmet in my case) I'm in no hurry to switch. There's no doubt that cheaper is possible, but will the quality still be there?
And when it comes to hardwoods then price is even less of an issue. Not because price isn't important. But because the cost of buying inferior materials far outweighs the benefit of shaving a pound or two off the price.
Nowadays I'm a full time furniture maker, but even as a hobbyist I took a similar approach. If I was sinking hundreds of hours of my time into a project I wasn't going to risk the outcome by looking for bargain basement timber. If I couldn't afford the material costs then I'd rather find a smaller and more affordable project than jeopardise the quality of the finished piece.
Just my two pennyworth.